Replace MBS? You say that like he is in am elected position. He's the heir to an absolute monarchy. Unless by replace, you mean a coup d'état of the Saudi Royal family?
There's no formal structure/rule for which prince leads / who is "closest" to the throne. It comes down to politics. MBS has many many siblings and cousins, essentially any of whom could "replace" him.
Saudi Arabia does not run by a primogeniture succession. Essentially all potential male heirs can become the overall leader. So all his brothers, his uncles, and some of his cousins are possible choices. It is politics that decides which one is heir aparent.
